1. Field of Invention:
This invention relates generally to light-weight high strength contourable cores for inclusion in a laminated contoured structure in which the core is sandwiched between facing skins, and more particularly to a core formed of a planar array of block-like modules held together in edge-to-edge relation by a scrim formed of elastomeric filaments which extend through the body of the modules in an intermediate plane parallel to the opposing faces of the core.
2. Prior Art:
The present invention, though applicable to various species of wood, is of particular value in connection with balsa wood derived from a tropical American tree (Ochroma pyramidale). Balsa wood has outstanding properties unique in the lumber field; for on the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength superior to any composite or synthetic material of equal or higher density. And while the invention will be described herein only in regard to balsa wood, it is to be understood that it is also applicable to many other wood species, as well as to rigid foam plastic and other materials having acceptable structural properties in the context of laminated structures.
The market for balsa wood is considerable, for structural sandwich laminates having an exceptionally high strength-to-weight ratio can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus, the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end-grain balsa wood. End-grain balsa-cored sandwich laminates are widely used in transportation and handling equipment, such as for floors for railroad cars, shipping containers, cargo pallets, bulkheads, doors, reefer bodies, as well as in a wide variety of cryogenic applications which exploit the excellent thermal insulation properties of balsa. These structural laminates are also employed in aircraft applications, in housing and in boating.
Where the structure to be reinforced is constituted by planar surfaces, the balsa core may simply be a solid board or panel laminated to the facings. But in the case of boat hulls and other structures having single and double curvatures or other complex contours, it is ordinarily not possible to conform solid balsa to the contoured surface without bending the balsa panel. Such bending involves difficult, time-consuming and expensive procedures.
Contourable balsa blankets are now commercially available that are composed of small individual balsa blocks attached to a common carrier such as a fabric scrim, whereby the blanket may readily be conformed to a curved surface for lamination thereto. A contourable balsa wood core of this type is disclosed in the Shook U.S. Pat. No. 3,540,967 and is marketed under the trademark "Contour Kore" by the Baltek Corporation of Northvale, N.J.
Such contourable balsa blankets are useful in the construction of reinforced plastic boats and larger vessels, for they lend themselves to lamination between layers of resin-reinforced fiberglass or other plastic material, thereby bringing about a distribution of weight favorable to high stability and buoyancy, as well as imparting stiffness to the structure.
The above-identified Shook patent discloses a contourable blanket in which the end-grain balsa wood blocks are attached by pressure-releasable adhesive lines to a fabric scrim made of a non-woven, open-mesh material of fiberglass or other non-stretchable yarns of high tensile strength. Because of the open mesh, the surfaces of the blocks are almost fully exposed to facilitate lamination. On the other hand, the stability of the scrim maintains the balsa blocks or modules at their assigned positions in the planar array thereof and prevents overlapping thereof during handling. Because fiberglass is wettable, it may be effectively bonded by standard resins and other adhesive agents both to the blocks and to the laminating plies.
When the contourable core is laid down on a contoured surface for lamination thereto with the scrim facing out, and the blocks are then pressed into conformity with the surface by a roller or other means, each block will assume an orientation determined by the surface engaged thereby. Where the orientation of a given block varies from that of the adjacent block as will occur in complex curvatures, the block will then partially detach itself from the scrim to the extent necessary to assume the desired orientation. Thus for a given contoured surface, selected blocks in the core are detached partially from the scrim, such detachment being facilitated by the adhesive line connections which permit release when pressure is applied to the block.
It has been found in working with a contourable blanket having a scrim secured to one face thereof, that bonding of the scrim-free face to a facing skin can be rendered highly effective, whereas the bonding of the scrimmed face to a facing skin is rendered somewhat less effective in that the scrim lies within and degrades the bonding interface. Since the strength of the resultant laminated structure depends in good part on the strength of the bonds between the core and the facing skins, the relative weakness of the scrim-side bonding line represents a negative factor in this regard.
Another factor which militates against the effectiveness of an exposed scrim blanket is that the scrim, being attached to the balsa wood blocks on one face thereof, creates an unbalanced structure; for if the wood swells because of a change in its moisture content, it is restrained only on the scrim face and not on the naked face. As a consequence, such blankets are subject to warping, which makes it difficult in some instances to hold it down on a resin-coated contoured surface when this coating is in the wet, uncured state.
Yet another drawback of an exposed scrim blanket of the prior art type is that the flexible scrim is non-stretchable; hence the blanket is conformable to a contoured surface whose curvature extends in one direction only. But if the contoured surface has a curvature running in the longitudinal direction as well as a curvature running in the transverse direction or is at some point concave and at another convex, the scrim blanket can be bent to conformity in only one of these directions, for the scrim then resists conformity in the other direction.